To limit switching losses in power semiconductor devices such as IGBTs (insulated gate bipolar transistors) and the associated recovery diode, JFETs (junction field effects transistors), power MOSFETs (metal oxide semiconductor field effect transistors) and power diodes, a field stop zone is typically built into the construction from the rear side of the die. Accordingly, the low impedance base, which accommodates the blocking layer, is reinforced with doping at the surface so that further expansion of the electric field is suppressed when the blocking voltage is increased.
To ensure the rear side emitter (for instance with an IGBT) is still sufficiently efficient, the total amount of the electrically-active field stop zone for the breakdown charge must not exceed approximately 1·1012 cm−2. Conventional field stop zones have a typical penetration depth of about 10 to 20 μm, the typical surface concentration occurring with diffusion of a doping substance such as phosphorous implanted for example with ion implantation approximately at 1015 cm−3 during a high temperature process.
Since an n-doped high impedance base material is typically used to form field stop zones for power semiconductor devices, the donor is usually employed as a doping substance. Standard pentavalent elements which are used for n-doping such as phosphorus, arsenic or antimony are typically used as the field stop implant. These elements, however, have the disadvantage that in order to achieve diffusion at depths of 10 to 20 μm, a relatively high thermal budget (time, temperature) must be used. However, high thermal budgets are not economical or compatible with the superstructure device formed at the front side of the die. Improved dynamic characteristics are also required during a further development of such structural elements in order to further increase the depth of the field stop zone e.g. to a depth of 50 μm or more.
Chalcogens such as sulfur, selenium and tellurium have been employed to fabricate field stop zones for power semiconductor devices. Chalcogens belong to group 16 of the periodic table of elements and have the effect of double donors. Chalcogen elements also have a higher diffusion constant than pentavalent elements, so that already at moderate process temperatures approximately between 900° C. and 1,000° C., penetration depths up to 30 μm can be realized. These types of field stop zones can be sufficient for blocking voltages up to about 600V. However, deeper penetrating field stop zones are needed in order to block higher voltages e.g. 1200V and above.
Phosphorous doping is also widely employed to provide base n-type doping for semiconductor substrates such as silicon wafers prior to device fabrication. For example, an n-doped float zone base material is used as starting material whereby the specific resistance is adjusted during the crystal growth. As an alternative, neutron radiation is carried out with a starting material which has very high impedance, whereby silicon is converted into phosphorus with a nuclear reaction with so called neutron transmutation doping (NTD). Due to a small capture profile for the neutrons, this NTD yields a very homogenous doping throughout the Si member. Radial resistance fluctuations can be greatly reduced, which means that the material can be used for applications in which high voltages are employed.
However, the application of float zone materials has disadvantages. For example, the application of float zone materials is relatively expensive and imposes limits on the size of the wafer which can be used. On the other hand, it is significantly cheaper to use Czochralski material, which can be manufactured by drawing from a crucible, and which can yield larger diameter wafer for memory or logic structural components. Nevertheless, due to a high reactivity of silicon, the starting material is characterized by a high level of oxygen impurities (from air) and also of carbon (from the crucible material). These impurities which occur mostly in the form of oxygen precipitates are removed with diffusion through a suitable thermal treatment above 1,000° C. in deep layers of the wafer, wherein a so-called denuded zone (DZ) is formed on the side which is mostly free of impurities. This zone is used mostly for the manufacturing of lateral structural components. Standard CZ material typically has a DZ depth of 10-20 μm and is adequate for memory and logic structural components.
However, if the material is used for the manufacturing of power semiconductor components which have a vertical superstructure, the depth of the DZ must be adjusted to match the length of the drift zone. Accordingly, the DZ must be extended for the voltage range of 400 V to 1,200 V with a depth of at least 40 μm to 120 μm. For the above mentioned reasons relating to costs and ratios, it is very desirable when such a starting material is used also for the manufacturing of power semiconductor devices such as e.g. IGBTs, JFETs, power MOSFETs and diodes. After the cell structure has been formed on the front side (e.g. DMOS cells, anodes, etc.), the remaining precipitate-rich material is then carried away from the DZ so that the remainder of the processing is performed from the rear side of the device. This includes for example the introduction of the field stop zone or of an emitter on the rear side by using a sufficiently low thermal budget.
As such, a CZ material which has a sufficiently high DZ depth is typically used for semiconductor devices. Such a material is offered under the label “Magic Denuded Zone” (MDZ) by the Monsanto Electronic Materials Company (MEMC) with wafer diameters of 6″, 8″ and 12″. With suitable RTP (rapid thermal processing), the crystal is strongly oversaturated in empty position locations so that a sink is provided in a sufficiently large depth for the diffusion which removes the oxygen. Another possibility is the use of magnetically drawn CZ material (MCZ). The crystal growth process during which oxygen is also implemented in the crystal is in this case reduced by a magnetic field and the development of oxygen precipitates is thus prevented. One problem, however, exists with respect to the relatively low tolerance which is created for the concentration of the doping material of the starting material for employment in power semiconductor devices.
Radial variations of the specific resistance are in this case due to the manufacturing process which is used for the CZ material in the range from several to more than 10%. This is caused by radial fluctuations (striations) of the doping substance, which in turn are due to the currents existing in the fluid phase and the variations that are created throughout the wafer with the segregation of the doping substance in the melt. Doping variations in the vertical direction of the Si rod can be much more severe. On the other hand, the usual specification for the FZ material, which is used as a standard, allows only for a variation of ±15%.
The variations of the raw values can be reduced when a starting material is used which is only slightly doped and the material is then adjusted using a targeted proton irradiation. However, very high energy levels are required with an increased drift zone in order to guarantee full penetration of the radiation.